Foldl on string

Question

I would like to foldl a string so that any occurrence of zero which is preceded by @ is replaced with "k". So "[email protected]" becomes, "[email protected]". How do I do that? So far my attempt is

test = foldl(\x acc-> if((last acc) == "@" && x == "0" then acc ++ "k" else acc ++ x)) "" "[email protected]"

However, it doesn't work. Foldl is expecting list of string and what I'm providing is just a string. How do I overcome that?

Answer 1

Taking chi's advice,

rep "" = ""
rep ('@' : '0' : xs) = "@k" ++ rep xs
rep (x : xs) = x : rep xs

If we want to get fancier,

rep = snd . mapAccumL go False
  where
    go True '0' = (False, 'k')
    go _ '@' = (True, '@')
    go _ x = (False, x)

or even

rep = snd . mapAccumL go 'x'
  where
    go '@' '0' = ('0', 'k')
    go _ y = (y, y)

To use foldr with this second approach (just because it's shorter; the first will work fine too, and allows generalization),

rep xs = foldr go (const "") xs 'x'
  where
    go '0' r '@' = 'k' : r '0'
    go x r _ = x : r x

To use zipWith (which is more awkward to generalize):

rep xs = zipWith go ('x' : xs) xs where
  go '@' '0' = 'k'
  go _ x = x